In 2010, the United Nations Framework Convention on Climate Change (UNFCCC) set an international policy goal to keep global warming below 2°C to limit the dangerous impacts of climate change. Some of the impacts that may occur if we exceed a 2°C increase in Earth’s average surface temperature include:

Increased risk of extinction for 20-30% of plant and animal species worldwide

Significant risks to many unique and threatened ecosystems

Substantial loss of coral reefs

20% reduction in freshwater availability and significant and widespread droughts

70-100cm sea level rise by 2100

Reduced crop yields in sub-Saharan Africa and south Asia

Examples of impacts associated with global average temperature change from IPCC AR4. Impacts will vary by extent of adaptation, rate of temperature change and socio-economic pathway). http://www.ipcc.ch/publications_and_data/ar4/syr/en/figure-3-6.html

Even below 2ºC, there can be significant impacts from global warming. Over 100 nations (mostly developing) have appealed to the U.N. that they could not survive in a world that is 2°C warmer and called for the policy target to be lowered to below 1.5°C. The 2014 IPCC assessment confirmed that the evidence is now overwhelming that 2°C of warming is intolerably dangerous due to projected disastrous or catastrophic impacts on human populations and ecosystems, but as we approach the 21st U.N. climate negotiations in Paris this December, a 2ºC limit remains a key international goal.

A Climate Friday FAQ reader’s question

I was delighted when Climate Friday FAQ reader, Jonathan Victory, sent me his climate science question last week: “Even if we stay under 2ºC, could that trigger feedback loops that send us over?” It’s clear that Jonathan already has an understanding of climate science based on his awareness of the 2ºC limit and the concept of feedback loops. For the rest of the blog-reading world, who may not be familiar with the concept of a feedback loop, I’ll explain before I answer Jonathan’s question.

What are feedback loops?

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Feedbackis the process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first (i.e. loop). Feedback processes may increase or decrease the effect of the forces that affectclimate (e.g. increased greenhouse gas emissions). One example of a climate feedback is when global warming caused by carbon dioxide emissions increases evaporation from water bodies and thus increases the amount of water vapour in the atmosphere. Since water vapour is a greenhouse gas itself, this increased evaporation in turn leads to further global warming.

Modelling all the Earth’s feedback processes to make predictions about future climate is extremely complicated. Climatemodels to date include “fastfeedbacks”, which are processes that act on an immediate time scale, such as evaporation and water vapour formation. “Slow feedbacks” (e.g. the loss of the cooling effect due to melting of ice sheets causing further warming) are slow to respond to warming. Since climate models don’t go beyond 100-200 year predictions, slow feedbacks are not included in IPCC global warming models. When slow feedbacks do finally respond to warming, they can lead to catastrophic acceleration ofwarming.

“Even if we stay under 2ºC, could that trigger feedback loops that send us over?”

In 2013, James Hansen and 17 other technical experts published a call to abandon the 2ºC target and aim to keep warming below 1ºC. One of the main arguments that Hansen and associates employed was that a 2ºC temperature increase would spur “slow feedbacks and eventual warming of 3–4°C with disastrous consequences”.Excluding slow feedback processes was appropriate for climate models of the last century when conditions were more stable. However, in order to predict warming for the 21st century and beyond, Hansen argued that slow feedback processes must be considered because their “instigation is related to the danger of passing ‘points of no return’, beyond which irreversible consequences become inevitable, out of humanity’s control”.

These ‘points of no return’ that Hansen refers to are also called ‘tipping points’ because, like the tipping over of a wine glass, they move the earth from one stable state (upright and full glass) to another stable, yet irreversible, state (tipped over, spilled glass). One example of a tipping point in climatology is the melting of the Greenland ice sheet – When the earth’s temperature reaches a certain level of warming (somewhere between 1-4ºC), the entire melting of the Greenland ice sheet will be “locked in” and eventually result in a global sea level rise of over 7m. Such an event would be irreversible and catastrophic.The short answer to Jonathan Victory’s excellent question is yes. - Even if we keep our greenhouse gas emissions low enough to stay under 2ºC, feedback loops could cause temperatures to increase further and send us over 2ºC. Our climate models do not account for slow feedbacks and some of the fast feedback modelling (e.g. cloud formation) is still uncertain. If we aim for 2ºC limit based on current models, in the long term (after 2100) the slow feedback processes could result in further warming of 4ºC or more, which would make life very difficult for those who have to try to exist on such a planet.

What does this mean for us?

People's Climate Ireland and Young Friends of the Earth campaigning on Global Divestment Day 2015

Question:given your reply to Jonathan's question, is it feasible at this stage to limit warming to under 1 degree even if we stopped emitting ghgs straight away? I. E. Are there enough ghgs in the atmosphere already to trigger dangerous and irreversible climate change?